Testing is performed on semiconductor devices to locate defects and failures in such devices, typically occurring during the manufacture of the semiconductor devices. As circuit density on semiconductor devices increases, the number of defects and failures can increase. Semiconductor manufacturers, therefore, have an increasing need to detect defects and failures in semiconductor devices as circuit density increases.
Thus, for quality control and to improve yields of semiconductor devices, semiconductor devices are tested, often before a die containing the semiconductor device is packaged into a chip. A series of probes on a test station electrically contact pads on each die in a wafer to access portions of the individual semiconductor devices on the die. For example, in a semiconductor memory device, the probes contact address pads and data input/output pads to access selected memory cells in the memory device. Typical dynamic random access memory devices ("DRAM") include one or more arrays of memory cells that are each arranged in rows and columns. Each array of memory cells includes word or row lines that select memory cells along a selected row, and bit or column lines (or pairs of lines) that select individual memory cells along a row to read data from, or write data to, the cells in the selected row.
During testing, predetermined data or voltage values are typically written to selected row and column addresses that correspond to certain memory cells, and then the voltage values are read from those memory cells to determine if the read data matches the data written to those addresses. If the read data does not match the written data, then the memory cells at the selected addresses likely contain defects and the semiconductor device fails the test.
Nearly all semiconductor devices, particularly memory devices, include redundant circuitry on the semiconductor device that can be employed to compensate for certain detected failures. As a result, by enabling such redundant circuitry, the device need not be discarded even if it fails a particular test. For example, memory devices typically employ redundant rows and columns of memory cells so that if a memory cell in a column or row of the primary memory array is defective, then an entire row or column of redundant memory cells can be substituted therefor, respectively.
Substitution of one of the spare rows or columns is conventionally accomplished by opening fuses (or closing antifuses) in a row or column decoder on the die. Conventional fuses include polysilicon fuses which can be opened by a laser beam, and also avalanche-type fuses and antifuses. If a given row or column in the array contains a defective memory cell, then the wafer can be moved to another station where a laser blows a fuse to enable a redundant row or column. The redundant row or column is then accessed simply providing the address to a row of address decoder that substitutes the appropriate redundant row/column for the defective row or column in the primary memory array.
The rows and columns of redundant memory cells, as well as the primary memory cells, must be tested. During the testing process, additional pads are provided on the die so that probes on the test station can write data to and read data from these redundant rows and columns.
A semiconductor device can be most thoroughly tested when the device is still in die form on the semiconductor wafer. The series of tests performed on the semiconductor device while in die form are typically performed in a sequential manner where initial tests attempt to weed out dies that may fail under future tests. These dies are typically discarded as being fatally defective at an early stage because it is time-intensive, and therefore expensive, to thoroughly test such dies if initial test results of the die conclude that the device will likely fail. Furthermore, it is even more expensive to prepare and package a die into a chip. As a result, it is desirable to determine at an early stage which dies will fail as acceptable packaged chips.
For example, if the test of the die shows that some of the redundant rows and columns contain defects, then the die may be discarded since more thorough testing downstream may indicate failures for which an insufficient number of redundant elements can be employed. Therefore, the decision is made to mark a given die as defective, and therefore not incur the expense of preparing, packaging and testing the chip which will likely fail during downstream testing. A small margin of devices, however, could pass future tests, and therefore be commercially acceptable (e.g., only a few defects on the device are discovered and therefore only a few redundant rows/columns are necessary). Therefore, it would be desirable to package and test nearly every die to thereby produce a greater number of commercially acceptable chips. However, packaged chips cannot be as thoroughly tested as unpackaged dies in wafer form. Therefore, it would be desirable to be able to fully test packaged chips in the same manner as unpackaged dies.
Another drawback of testing semiconductors in wafer form is that semiconductor wafers are often difficult to manipulate, and typically require a test bed or other apparatus to releasably secure the wafer while the probes are moved to contact the pads on each die. Additionally, moving the wafer from one test station to another (e.g., to a second station having a laser for blowing fuses), adds to the time-consuming nature of testing semiconductor devices in die form. Overall, the testing of semiconductor devices in die form is time consuming. Therefore, semiconductor manufacturers desire to test a given semiconductor device after it has been packaged as a semiconductor chip, because the chip can be automatically inserted into a test socket using pick and place machinery. Automated testing circuitry can then apply predetermined voltages and signals to the chip, write test patterns thereto, and analyze the results therefrom to detect for failures in the chip.
Often, the number of pads on a die is greater than the number of pins on the packaged semiconductor chip. Therefore, as noted above, certain tests performed while the semiconductor device is in die form cannot be performed on the device after it has been packaged. For example, redundant rows/columns can be directly accessed when the device is in die form, but not when packaged into a chip. As a result, packaged chips necessarily undergo less rigorous testing than unpackaged dies. Packaged chips can include manufacturing defects that are not yet failures and thus are undetectable by the limited number of tests capable of being performed on the packaged chips.
To provide for rapid testing of chips, and to compensate for the limitation on the number of pads accessible in a packaged chip, manufacturers currently employ on chip test mode circuitry that initiates a special test mode when a predetermined combination of signals or "test key vector" signals are applied to the pins of the chip. For example, external test circuitry can force a low value to a write enable pin WE before signals applied to column address select ("CAS") pin and row address select ("RAS") pin fall to a low value. The test key vector signals applied to the pins are preferably selected to be signals not normally applied to such pins by a user under normal operation of the chip, and therefore, a user would not accidentally enter the test mode for the chip. To further ensure that a user does not accidentally enter into a test mode for the chip, a continuous supervoltage (e.g., 10 volts) must be applied to a pin on the chip to enter the test mode. Such a supervoltage is one beyond the normal tolerances for the chip and therefore would not normally be applied to the chip by a user under normal conditions.
For example, the redundant rows/columns can be accessed by applying a certain test key vector signal and a supervoltage to the chip, which allows the chip to enter into a particular test mode. The redundant rows and columns can then be tested while the die is in its packaged chip form, even though a particular pad on the die is not directly accessible.
While it is desirable to test the redundant rows and columns after the die has been packaged as a chip, the currently known methods for accessing and testing the redundant rows/columns is time-consuming. External testing circuitry, after accessing and testing the rows and columns of cells in the primary memory array, must apply a new test key vector signal and supervoltage of the chip to access the redundant rows and columns. The need to apply the test key vector signal and supervoltage adds additional steps during the testing process which necessarily increases the time required to test the chip.
Additionally, current memory devices divide the primary array of memory cells into sub-arrays, or blocks, so that only a portion of the memory need be energized in a given access, resulting in significant power reduction. As the number of sub-arrays or blocks increases, e.g., up to 16 or 32 blocks, each block typically includes its own redundant rows and columns. As a result, conventional testing of the redundant rows and columns requires the external testing circuitry to frequently enter into and out of the special test mode for accessing the various redundant rows/columns. In other words, the external test circuitry must frequently and repeatedly apply the test key vector signal and supervoltage for each redundant row and column in the multiple blocks, which thereby further increases the time-consumptive nature of such testing.